1. Field of the Invention
The present invention relates to an electroluminescent device (referred to simply hereinafter as an "EL device") for use in, for example, emissive type segment displays and matrix displays of instruments, displays of various types of information terminal machines, and the like. The present invention also relates to a process for fabricating the EL device.
2. Related Art
EL devices fabricated heretofore utilize the light-emitting phenomenon of a luminescent layer. More specifically, it uses the light emission by applying an electric field to a luminescent layer comprising an element acting as a luminescent center in a host material based on a Group II-VIB compound such as zinc sulfide (ZnS). EL devices are attracting much attention as components for implementing an emissive type panel display.
A cross sectional view of the structure of a generally used EL display 10 is shown in FIG. 3.
In FIG. 3, the EL device 10 comprises a glass substrate 1 as an insulating substrate, having thereon sequentially stacked layers of a first transparent electrode (first electrode) 2 comprising an optically transparent ITO (indium tin oxide) film, etc.; a first insulating layer 3 comprising tantalum pentaoxide (Ta.sub.2 O.sub.5), etc.; a luminescent layer 4; a second insulating layer 5 comprising tantalum pentaoxide (Ta.sub.2 O.sub.5), etc.; and a second transparent electrode (second electrode) 6 comprising an optically transparent ITO film, etc.
An ITO film is a transparent electrically conductive film based on a tin(Sn)-doped indium oxide (In.sub.2 O.sub.3), and it has been used widely as a transparent electrode.
The luminescent layer 4 may comprise zinc sulfide (ZnS) as a host material with an element such as manganese (Mn), terbium (Tb), or samarium (Sm) incorporated therein as a luminescent center. Otherwise, it may comprise strontium sulfide (SrS) as a host material with cerium (Ce) incorporated therein as the luminescent center.
The color of a light emitted by an EL device 10 depends on the combination of the host material and the element that is added as the luminescent center. Thus, in case of using zinc sulfide (ZnS) as the host material, an amber-emitting phosphor can be obtained by adding manganese (Mn) as a luminescent center; a green-emitting phosphor is realized by adding terbium (Tb), and a red-emitting phosphor is achieved by adding samarium (Sm). When strontium sulfide (SrS) is used as the host material, a blue-green emission can be obtained by adding cerium as the luminescent center. A blue-emitting phosphor can be obtained when cerium is added into calcium thiogallate (CaGa.sub.2 S.sub.4), strontium thiogallate (SrGa.sub.2 S.sub.4), or barium thiogallate (BaGa.sub.2 S.sub.4).
An unexamined Japanese Patent Publication H5-65478, for instance, discloses the effect of the concentration of cerium in the luminescent layer and the quantity of adding gallium sulfide (Ga.sub.2 S.sub.3) into the sputtering target on increasing brightness of a blue-emitting EL device utilizing calcium thiogallate (CaGa.sub.2 S.sub.4), or strontium thiogallate (SrGa.sub.2 S.sub.4). However, it was is found that the brightness was still insufficient.
An EL device emits light when accelerated electrons collide with the luminescent center. More specifically, the electrons inside the luminescent layer or the electrons that are injected into the layer from the boundary between the luminescent layer and the insulating layer are accelerated by an AC voltage applied to the electrodes provided on both ends of the luminescent layer, and are collided against the luminescent center. Thus, the brightness can be increased with an increasing number of electrons accelerated for exciting the luminescent center.